In active power switching applications, a decision/controller circuit is provided to control two switches to select between one of two power supplies having the higher voltage. These decision/controller circuits typically are connected to a power terminal on the output of the two switches to receive the operating power therefrom. In operation, these decision/controller circuits receive as comparison inputs the voltages output from each of the power supply. Two control outputs are provided to control each of the two switches, depending upon the relative input levels.
One disadvantage to the present active decision/controller circuits is that they are powered from the output side of the switches. This presents a disadvantage during a power-up situation wherein no power supplies are initially present, or there is an insufficient power level on either of the supply inputs.
In the power-up or low power condition, the decision/controller circuit is essentially inoperative until the switches close to provide power thereto. This is due to the fact that the control for the switches requires power to be applied to the decision/controller circuit prior to either of the switches being activated. Controller lockout has been observed in previous implimentations. This condition is due to the combination of finite source impedance and the existence of parasitic bipolar transistors which, in a power up condition, tend to clamp the output node to ground, rendering the controller inoperable. One solution has been to attempt to favor one of the switches in order to force the power to be steered in one direction. In view of the above disadvantages, there exists a need for a power steering circuit that receives its operating power from a supply different from the output of the switches in a power steering circuit for at least the power-up condition in order to make a valid decision.